Harvard University
Harvard Writing Center: Tips on Grammar, Punctuation and Style
Rather than lists of rules, this site provides practical advice on the use of commas, semicolons, dashes, hyphens, abbreviations, acronyms, split infinitives, "this," "that," italics, and underlining. CCSS.ELA-Literacy.CCRA.L.2
English Plus+
English Plus: Grammar Slammar!
An easy-to-use grammar resource containing rules for style, usage, capitalization, abbreviations, punctuation, and letter writing. This site also includes a glossary of common grammatical mistakes.